Over the years numerous modifications in diode-pumped laser design have been made to increase performance in terms of the output power. Recently, the trend in longitudinal or end pumping of the active gain medium has been to provide for greater output power as compared to earlier pump configurations; however, excessive pumping levels in any orientation might create the problems associated with heating of the gain medium. Longitudinal pumping of Nd:YAG lasers by laser diodes is generally preferred over transverse pumping for efficient TEM.sub.00 operation owing to the overlap of the resonator mode with the inversion profile produced by the pump beam. A limiting factor is that the scaling of longitudinally pumped lasers has been constrained by the power available from single stripe diodes. Current technology limits the maximum pump power along a single axis to about 6 W by polarization combination of two 3 W laser diodes. Recent developments in scaling have attempted to address the problem of providing higher longitudinal pump power and have included techniques that, 1.) utilize incoherent arrays for longitudinal pumping, see S. C. Tidwell et al.'s "Scaling Output Power of End-Pumped Solid-State Lasers", Conference on Lasers and Electro-Optics, Paper CFC1, May 17, 1991, Baltimore, 2.) have resonator designs that allow pumping simultaneously along two axes, see R. Scheps et al.'s "Scalable Single Frequency Diode-Pumped Ring Laser", Appl. Opt., Vol 31, No. 9, Mar. 20, 1992, pp 1221-1224, 3.) have zig-zag slab lasers pumped with linear arrays, see D. Krebs' "Diode-Pumped Slab Laser Study", McDonnell Douglas Technical Report N66001-83-C-0071 (April, 1986); and T. M. Baer et al.'s "High Peak Power Q-switched Nd:YLF Laser Using a Tightly Folded Resonator", Conference on Lasers and Electro-Optics, Paper CMF2, May 21, 1990, Anaheim, and 4.) have resonators containing multiple gain elements, see R. Scheps et al.'s "A Single Frequency Nd:YAG Ring Laser Pumped by Laser Diodes", IEEE Quantum Electron, vol. 26 (1990), pp. 413-416. Additional scaling developments have been based on combining the output of several laser diodes to form a single pump beam, see R. Scheps et al.'s "Alexandrite Laser Pumped by Semiconductor Lasers", Appl. Phys. Lett, vol. 56 (1990), pp. 2288-2290 and T. Y. Fan et al.'s "Scalable, End-Pump, Diode-Laser-Pumped Laser", Opt. Lett, vol. 14 (1989), pp. 1057-1059.
While these approaches to scaling are promising, the need for a simple resonator capable of producing several Watts of TEM.sub.00 power has led to a search for alternative designs. A straightforward concept based on a folded hemispherical resonator, such as that mentioned in the article "Highly Efficient Neodymium: Yttrium Aluminum Garnet Laser End Pumped by a Semiconductor Laser Array" by D. L. Sipes in Appl. Phys. Lett., Vol. 47 (1985), pp. 74-76, can accommodate two pump axes and hence allows for a doubling of the pump power. The term "folded resonator" in general refers to a laser resonator where all the internal components do not lie in a single straight optical axis. Instead, there is a discrete flat "fold" mirror in the resonator that reflects the resonator radiation along a second linear optical axis, usually oriented 90.degree. with respect to the first linear optical axis. In such a device the pump radiation incident on the interior face of the laser crystal typically penetrates the fold mirror, but in practice pumping through the fold mirror requires a long focal length lens. Consequently, too large a pump waist is produced at the rod which results in reduced efficiency.
Another recent design is the zig-zag slab laser configuration such as that disclosed in U.S. Pat. No. 4,785,459. A confocal resonator design is transversely pumped and bounces a resonator mode beam in a zig-zag path from side to side of a slab gain medium. The gain medium should have its sides perfectly parallel and is said to be polished to about .lambda./2. At each bounce point a stripe of a multistripe laser bar is precisely located and positioned to hopefully provide a cumulative pumping power without introducing unwanted influences to thereby enhance the laser output. The resonator mode size throughout the zig-zag configuration is said to be about 200 microns. Fabrication of the slab appears to be complicated and, as a consequence, may be expensive.
Thus, a continuing need exists in the state of the art for a simple compact laser design that can be longitudinally pumped to provide several watts of CW output power. Such a design is one that employs a multi-faceted gain medium that internally folds a hemispherically-shaped resonator mode radiation. At each internal fold face, the gain medium may be optically pumped by a number of pumping mode radiations emitted from a plurality of appropriately oriented pumping mode sources to allow increased pumping levels and increased laser output.